1. Field of the Invention
The present invention relates to a small-sized electronic instrument and particularly to the improved structure of a small-sized electronic instrument such as an electronic wrist watch or the like which utilizes a plastic base plate.
2. Description of the Related Art
Conventional small-sized electronic instruments, for example, electronic wrist watches have a base plate of metal material such as brass which can meet the dual requirements of sufficient strength and easy workability. FIG. 5 is a fragmentary cross-section of the assembled structure of a wrist watch mechanism having a metallic base plate.
In FIG. 5, the metallic base plate designated by reference numeral 10 includes a threaded tube 11 of metal press fitted thereinto from the bottom face of the base plate. The threaded tube 11 is used to position and tighten various parts relative to the base plate 10. The threaded tube 11 has a flange 11a formed therein at the bottom end, the flange 11a serving as a stop for preventing the threaded tube 11 from moving beyond the top of the base plate 10.
In the electronic wrist watch illustrated, the threaded tube 11 functions to position and hold a stator 12, a coil core 13, a coil terminal sheet 14, a composite circuit 15 and a gear train bridge 16 in the order described. These five parts are tightened together relative to the threaded tube 11 by a setscrew 17 threaded into the threaded bore of the threaded tube 11.
In such an arrangement, therefore, a number of parts can be assembled and held together by a few screws. This is advantageous particularly in the automated assembly process since the time and cost required to assemble various parts can be greatly reduced.
Recently, the utilization of molded plastics is increasing in such small-sized electronic instruments. The base plate is also frequently made of a plastic material such as engineering plastic or the like. However, there is still frequently used metallic threaded tubes each of which is press fitted into or molded into the base plate for providing sufficient strength and reliability in the certain part that will be subjected to a relatively large torque or a relatively large force exerted on the threaded tube in the direction of withdrawal.
In spite of the presence of this strength problem, a structure consisting of a base plate and threaded tube which are molded into a unit made from a plastic material is highly desired in the art for such reasons that a self-tapping screw can be used therein and that the manufacturing cost can be reduced. An example of such a base plate generally used in an electronic wrist watch is designated by reference numeral 20 in FIG. 6. The base plate 20 is made of a plastic material such as polycarbonate or the like and includes a sleeve-like screw housing portion 20a integrally molded therein at the positioning and holding location. As in the prior art of FIG. 5, around the outer periphery of the screw housing portion 20a are stacked a stator 21, a coil core 22, a coil terminal sheet 23 and a composite circuit 24, these parts being then firmly held relative to the screw housing portion 20a by a self-tapping type setscrew 25. As described, the screw housing portion 20a includes a bore 20b preformed centrally therethrough. The self-tapping screw 25 is threaded into the bore 20b of the screw housing portion 20a while tapping the bore 20b.
The structure of the base plate 20 with the screw housing portion 20a integrally molded therein serves to reduce the working or assembling cost. On the other hand, there is another problem in which the strength of the screw housing portion 20a is relatively low, leading to easy damage of the threads when tapping. More particularly, the polycarbonate material used to form the base plate 20 is relatively flexible and has its bending modulus of elasticity equal to about 50,000 Kg/cm.sup.2. Therefore, such a plastic material may be easily damaged even if a relatively small torque such as about 250 g-cm is applied thereto as in the automated assembly process. In the automated assembly process, therefore, the tightening torque must be strictly controlled.
In addition, the damage of the threads in such tube members is frequently caused by repeated disassembly and assembly when repairing and overhauling small-sized electronic instruments.
Attempts have been made to overcome these problems by replacing the polycarbonate material with harder polyphenylene sulfide material or polythioether sulfone material. These hard plastic materials have a bending modulus of elasticity equal to about 100,000 Kg/cm.sup.2, which is two times greater than that of said polycarbonate. Thus, threads are not damaged when self-tapping.
On the other hand, these hard plastic materials raise another problem in that the screw housing portion of the base plate made of the hard plastic material is easily cracked or broken at its root. The base plate of hard plastic material requires an increased wall thickness or any other reinforcing means at its screw housing portion. This increases the dimensions of the electronic instruments.